The present invention relates generally to communications over a shared-access data network, and more specifically to a technique for implementing IP subnet sharing among multiple network devices without using bridging or routing protocols.
Broadband access technologies such as cable, fiber optic, and wireless have made rapid progress in recent years. Recently there has been a convergence of voice and data networks which is due in part to US deregulation of the telecommunications industry. In order to stay competitive, companies offering broadband access technologies need to support voice, video, and other high-bandwidth applications over their local access networks. For networks that use a shared access medium to communicate between subscribers and the service provider (e.g., cable networks, wireless networks, etc.), providing reliable high-quality voice/video communication over such networks is not an easy task.
One type of broadband access technology relates to cable modem networks. A cable modem network or “cable plant” employs cable modems, which are an improvement of conventional PC data modems and provide high speed connectivity. Cable modems are therefore instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services.
FIG. 1 shows a block diagram of a conventional two-way hybrid fiber-coaxial (HFC) cable network 100. As shown in FIG. 1, the cable network 100 includes a Head End complex 102 typically configured to service about 40,000 homes. The Head End complex 102 may include a plurality of components and/or systems (not shown) such as, for example, a Head End, a super Head End, a hub, a primary hub, a second hub, etc. Additionally, as shown in FIG. 1, the Head End complex 102 typically includes a Cable Modem Termination System (CMTS). Primary functions of the CMTS include (1) receiving data inputs from external sources 100 and converting the data for transmission over the cable plant; (2) providing appropriate Media Access Control (MAC) level packet headers for data received by the cable system, and (3) modulating and demodulating the data to and from the cable network. Typically, the Head End complex 102 is configured to provide a communication interface between nodes (e.g. cable modems) in the cable network and external networks such as, for example, the Internet. The cable modems typically reside at the subscriber premises 110A-D.
The Head End Complex 102 is typically connected to one or more fiber nodes 106 in the cable network. Each fiber node is, in turn, configured to service one or more subscriber groups 110. Each subscriber group typically comprises about 500 to 2000 households. A primary function of the fiber nodes 106 is to provide an optical-electronic signal interface between the Head End Complex 102 and the plurality of cable modems residing at the plurality of subscriber groups 110.
In order for data to be able to be transmitted effectively over a wide area network such as HFC or other broadband computer networks, a common standard for data transmission is typically adopted by network providers. A commonly used and well known standard for transmission of data or other information over HFC networks is the Data Over Cable System Interface Specification (DOCSIS). The DOCSIS standard has been publicly presented by Cable Television Laboratories, Inc. (Louisville, Colo.), in a document entitled, Radio Frequency Interface Specification (document control number SP-RFIv2.0-I01-011231, Dec. 31, 2001). That document is incorporated herein by reference for all purposes.
Communication between the Head End Complex 102 and fiber node 106a is typically implemented using modulated optical signals which travel over fiber optic cables. More specifically, during the transmission of modulated optical signals, multiple optical frequencies are modulated with data and transmitted over optical fibers such as, for example, optical fiber links 105a and 105b of FIG. 1, which are typically referred to as “RF fibers”. As shown in FIG. 1, the modulated optical signals transmitted from the Head End Complex 102 eventually terminate at the fiber node 106a. The fiber nodes maintain the signal modulation while converting from the fiber media to the coax media and back.
Each of the fiber nodes 106 is connected by a coaxial cable 107 to a respective group of cable modems 112a residing at subscriber premises 110a-d. According to the DOCSIS standard, specific frequency ranges are used for transmitting downstream information from the CMTS to the cable modems, and other specific frequency ranges are used for transmitting upstream information from the cable modems to the CMTS.
In order to allow the cable modems to transmit data to the CMTS, the cable modems share one or more upstream channels within that domain. Access to the upstream channel is controlled using a time division multiplexing (TDM) approach. Such an implementation requires that the CMTS and all cable modems sharing an upstream channel within a particular domain have a common concept of time so that when the CMTS tells a particular cable modem to transmit data at time T, the cable modem understands what to do. “Time” in this context may be tracked using a counter, commonly referred to as a timestamp counter, which, according to conventional implementations is a 32-bit counter that increments by one every clock pulse.
FIG. 2 provides an example of a conventional DOCSIS enabled CMTS. In the embodiment shown in FIG. 2, the CMTS 204 provides functions on several layers including a physical layer 232, and a Media Access Control (MAC) layer 230. Generally, the physical layer is responsible for receiving and transmitting RF signals on the cable plant. Hardware portions of the physical layer include a downstream modulator and transmitter 206 and an upstream demodulator and receiver 214. The physical layer also includes software for driving the hardware components of the physical layer.
Upstream optical data signals (packets) arriving via an optical fiber node are converted to electrical signals by a receiver, and the upstream information packet is then demodulated by the demodulator/receiver 214 and passed to MAC layer block 230.
A primary purpose of MAC layer 230 is to encapsulate, with MAC headers, downstream packets and decapsulate, of MAC headers, upstream packets. The encapsulation and decapsulation proceed as dictated by the above-mentioned DOCSIS standard for transmission of data or other information. MAC layer block 230 includes a MAC controller 234 which is configured to provide the DOCSIS compliant functionality.
After MAC layer block 230 has processed the upstream information, it is then switched to an appropriate data network interface on data network interface 202. When a packet is received at the data network interface 202 from an external source, the packet is passed to MAC layer 230. The MAC controller 234 then transmits information via a one-way communication medium to downstream modulator and transmitter 206. Downstream modulator and transmitter 206 takes the data (or other information) in a packet structure and converts it to modulated downstream frames, which are then transmitted to downstream cable modems.
One of the problems associated with many conventional access networks relates to the scarcity of IP address space in such networks. Currently, service providers may use a variety of different techniques for sharing an IP subnet across multiple network devices in an access network. One such technique involves the subdividing of an IP subnet into multiple, smaller subnets. Each of the smaller subnets are then assigned to different network devices in the network. The network devices use standardized routing protocols to exchange the subnet information (within the network) with each other and with an aggregation router. The aggregation router reconstructs the original IP subnet for proper packet forwarding and routing with external networks.
One problem associated with the subdividing of IP subnets is that the IP subnet divisions are typically statically provisioned and assigned to specific network devices, and are not dynamically re-configurable. For example, using conventional techniques, it is possible to subdivide an IP subnet into multiple, smaller IP subnet groups wherein each IP subnet group is capable of providing, for example, 8 unique IP addresses. Assuming that each of the subnet groups were provisioned and assigned to a different DCMTS (in the cable network of FIG. 3), each of the DCMTS devices would be able to provide service for up to 8 different customers. However, if it were then desired for at least one of the DCMTS devices to provide service to more than 8 customers, the entire IP subnet subdivision must then be re-allocated using a new, statically allocated subnet subdivision scheme, and then provisioned out to each of the affected DCMTS devices in the cable network.
Another technique which is conventionally used for sharing an IP subnet across multiple network devices is referred to as route injection. Using the route injection technique, network devices, (such as, for example, DCMTS devices) may dynamically assign IP addresses to requesting customers on an individual basis, and inject the newly assigned host route into the access network using an aggregation gateway router. The aggregation router reconstructs the original subnet for proper packet forwarding and routing with external networks. However, in order to implement IP subnet sharing using the route injection technique, each network device must be configured to support and run a full suite of standardized routing protocols in order, for example, to exchange the injected host routes with other network elements and/or the aggregation router. Such a solution is therefore undesirable since it will result in a relative increase in cost and complexity of each network device.
Another technique which may be used for sharing an IP subnet across multiple network devices is through the use of a bridging technique, wherein the network devices are configured to handle traffic using layer 2 addresses (and not layer 3 addresses). Additionally, an aggregation router may be used to manage the IP subnet and communicate the IP subnet information with external networks. However, the bridging technique also presents a number of problems including excessive flooding issues, and more importantly, security issues.
Accordingly, it will be appreciated that there exists a continual desire to efficiently utilize addressing space in an access network in a manner which avoids at least some of the problems associated with conventional IP subnet sharing techniques.